Waterspouts

Last week, the Boon Companion and I were sipping sundowner cocktails in the British Virgin Islands, leaning back in our chairs, and cloud-watching. Long streets of fair-weather cumulus had been strung out over the Caribbean since midday. Now, half an hour before sunset, the cumulus was growing—boiling upwards to form towering cumulus congestus, which were draping little dark banners of rain here and there across the sunlit sea. At the end of a warm, still day, there was clearly some strong convection building out there on the water.

Quite suddenly, a little finger of grey cloud poked out of the dark base of one of the clouds. I watched it for a minute, as it grew thinner and longer, eventually stretching far enough to catch the rays of the setting sun. It was a funnel cloud. Which meant … Yes, on the sea horizon below the cloud there was a little swirling dark smudge—a spray ring. We were looking at a waterspout, the first I’ve ever seen.

As we watched, the funnel cloud sent down a thin dark streamer that speared into the centre of the spray ring—the waterspout was fully evolved. It stayed that way for about fifteen minutes, idling around gently beneath its parent cloud and slowly moving along the horizon, until it suddenly decayed away—the funnel withdrew into the cloud, and the spray ring collapsed. Show over.

There are actually two different kinds of waterspout. If a tornado moves out over water, it creates a tornadic waterspout. These are big fierce beasts, with all the destructive power of a tornado. They evolve in the fierce updrafts of supercell thunderstorms, and they form by dropping a thick funnel cloud downwards to the ground.

But what we saw in the British Virgin Islands was a fair weather waterspout. They’re generated by air rising into evolving cumulus clouds, and they progress from the surface upwards.

In their early stages, a swirl of wind forms around warm air rising from the water surface, just like a dust-devil spinning through a hot car-park. The wind swirl creates a characteristic disc of rough water with a calm centre—this is the “dark spot phase”, which is best seen from the air. A few spiral lanes of disturbed water then converge on the dark spot, and spray begins to rise into the air as the surface winds intensify. Above this visible spray ring, a column of spiralling, rising winds connects to the base of the cloud overhead. As the spiral tightens and intensifies, the pressure at its core drops by several millibars, and water vapour starts to condense within it—the cloud starts to extend downwards in a visible funnel cloud. In a fully developed waterspout, the funnel reaches all the way down to the centre of the spray ring, by which time it has usually developed a hollow centre—water droplets formed in the low-pressure core are flung outwards to orbit in the zone of highest winds, a few metres from the core.

Ocean Today have put together a nice two-and-a-half-minute video compilation that illustrates the evolution of a fair weather waterspout. It’s full of impressive images, and I recommend it. Here’s a link to the embedding version of the video, which seems reluctant to embed on this website:

Fair weather waterspouts are typically short-lived, persisting for about twenty minutes; and they often occur in groups of two or more when conditions are favourable. (Indeed, we spotted a couple of other tentative funnels appearing and disappearing at the cloud base while we watched our waterspout noodling around the horizon.) The Florida Keys reports more than a hundred a month between May and September, when the surface water temperature is over 25ºC.

They can be relatively benign—people have driven speedboats through them. But with wind speeds of 65 m/s (130 knots), they can also be very dangerous—large vessels have been capsized or dismasted; small vessels have been swamped by the spray ring; and in 1993 a windsurfer was drowned on the Chicago waterfront by a Lake Michigan waterspout.

The updraft is fierce—a waterspout that made landfall on Matecumbe Key, Florida, reportedly lifted a two-ton Cadillac a few feet into the air before setting it down again. So it seems at least possible that they can lift a significant quantity of water from the sea surface, and there are anecdotes to support this. In his book It’s Raining Frogs And Fishes, Jerry Dennis gives a report of a downpour of salty rain on the island of Martha’s Vineyard on August 19, 1896, a few hours after a waterspout had been seen close to shore—the salt water had presumably been incorporated briefly into the parent cumulus cloud. And in the Annals and Magazine of Natural History(January 1929), E.W. Gudger reported that a waterspout had dissipated near an open boat in the Gulf of Mexico, which was then promptly swamped under a torrent of seawater and fish.

Gudger’s article is entitled “More Rains Of Fishes”—he was primarily interested in sporadic reports of fish falling from the sky, and he felt that waterspouts offered a reasonable route by which aquatic creatures might get up there in the first place.

Surprisingly, falls of fish (and frogs) are quite well documented, both before and after Gudger’s time. For instance, on October 23, 1947, a Canadian fisheries biologist was fortuitously on hand to see fish falling from the sky in Marksville, Louisiana:

In the morning of that day, between seven and eight o’clock, fish ranging from two to nine inches in length fell on the trees and in yards […] There were spots on Main Street, in the vicinity of the bank (a half block from the restaurant) averaging one fish per square yard. Automobiles and trucks were running over them. Fish also fell on the roofs of houses.
They were freshwater fish native to local waters, and belonging to the following species: large-mouth black bass (Micropterus salmoides), google-eye (Chaenobrittus coronarius), two species of sunfish (Lepomis), several species of minnow and hickory shad (Pomolobus mediocris). The latter species were the most common.

Following Gudger’s example, waterspouts (and their terrestrial cousins, whirlwinds) are nowadays the usual explanation trotted out for such events—the fish or frogs are presumed to have been hoovered up from surface water, to bounce improbably around in the clouds for a while before falling on some unsuspecting town.

However, these anomalous falls tend to be oddly pure samplings, as in Bajkov’s description above, which seemed to involve nothing but fish. I suppose we can imagine a waterspout picking a shoal of surface fish out of the ocean quite cleanly; but what about the freshwater fish in Bajkov’s example? Or the frogs reported elsewhere? Why are such falls apparently never accompanied by mud, gravel, weeds and battered waterfowl, scooped up incidentally? I’ll give the last word on that topic to Charles Fort:

[…] a pond going up would be quite as interesting as a frog coming down. Whirlwinds we read over and over—but where and what whirlwind? It seems to me that anyone who had lost a pond would be heard from.